ML function

  Write a recursive ML function that uses pattern matching with name/type optconcat : char option list -> string that inputs a list of char options, and returns a single string consisting of all characters on the list concatenated together while ignoring any instances of NONE. For example, - optconcat([SOME #"b",NONE,SOME #"o",SOME #"o",NONE]); val it = "boo" : string - optconcat([NONE,NONE]); val it = "" : string